Single and multiple deck conveyor dryers for reducing the moisture content of various materials, including rigid and semi-rigid material in sheet form, such as, green veneer, wet plasterboard, fiberboard, perlite and bagasse matte and the like, wherein the material being dried is conveyed through a stationary housing on one or a plurality of tiered conveyors while heated gases are force circulated through the housing or a part thereof, are known. The increase in volume of the gas in the dryer incident to the evaporation of moisture from the material being dried is typically removed by one or more vents or ducts. In some systems, the exhaust is discharged directly to the atmosphere.
It has been found that in a typical dryer of this type, if the drying process is not carefully controlled and optimized, gases will be discharged through not only the exhaust stacks, but through the input and output ends of the dryer. Attempts have been made to control the inflow and outflow of gases through the input and output ends of a veneer drying apparatus. An example of one such attempt to improve the drying efficiency, is disclosed in U.S. Pat. No. 4,439,930, which is owned by the assignee of the present application. The assignee also owns U.S. Pat. No. 5,603,168, incorporated by reference and reproduced herein substantially in its entirety for ease of reference. The present invention is an improvement over the drying apparatus which is the subject of that patent.
As reported therein, it has been found desirable to control the flow of exhaust gases from a jet veneer drying apparatus, to not only optimize the drying efficiency of the dryer, but to also provide a means for containing and treating the exhaust gas prior to discharging to atmosphere. More specifically, it is now considered desirable to convey the exhaust from a jet veneer dryer to a volatile organic carbon (V.O.C.) alienating device such as a catalytic or thermal oxidizer prior to atmospheric discharge. In order to optimize the performance of this equipment it was disclosed as being desirable to maintain the temperature of the exhaust gas at or above a minimum operating temperature. However, pitch build-up, condensed V.O.C. material, in the exhaust fan duct of the wet end seal resulted from the lower exhaust stream temperature inherent to that design, which may also have been contributed to by improper setting of the exhaust control by dryer operators and insufficient maintenance and cleaning practices. Obviously, pitch build-up represents a fire hazard. A fire in the exhaust fan duct would result in costly repairs and downtime.